1. Field of the Invention
The invention generally related to anisotropic curable monomors and in particular to anisotropic diglycidyl compounds.
2. Description of the Prior Art
Anisotropic heat-curable monomers are known which can be cured to form anisotropic thermoset resins. It is known to those skilled in the art that anisotropic thermoset resins, frequently processed from "liquid crystalline" polymers or monomers, have desirable self-reinforcing characteristics. U.S. Pat. Nos. 4,440,945, 4,452,993, and 4,514,553 (issued to Conciatori et al.) and U.S. Pat. No. 4,683,327 (issued to Stackman) disclose anisotropic heat-curable acetylene-terminated and acrylic-terminated monomers which can be formed into self-reinforcing thermoset resins as a result of molecular orientation in the cross-linked resin. However, the tendency of these monomers to self-condense precludes copolymerization with other monomers which are not acetylene-terminated or acrylic-terminated. U.S. Pat. No. 4,654,412 (issued to Calundann et al.) discloses base anisotropic polyesters containing stilbene or tolan functionalities which are copolymerized by the addition of comonomers ("curing agents") such as maleic anhydride to form anisotropic thermoset resins. The principal of copolymerization per se allows for the preparation of a vastly larger number of advantageous variations from the base anisotropic resin, but the temperatures and viscosities of the base resins are disadvantageously high.
U.S. Pat. No. 4,764,581 (issued to Muller et al.) relates to diglycidyl compounds of the formula (I) ##STR1## and discloses anisotropic liquid crystalline diglycidyl compounds which may be cured with a desirably wide variety of comonomers to form anisotropic thermoset resins. Furthermore, these compounds may be processed at desirably low viscosities and temperatures. This invention therefore retains the advantages of copolymerization cited above, but improves upon previous inventions by allowing more desirable processing conditions. However, the diglycidyl groups of this invention were linked directly to the 4-hydroxyphenyl 4-hydroxybenzoate group, such that the liquid crystalline character of the example cited (R.sub.1 =R.sub.2 =R.sub.3 =R.sub.4 =H) was only observed on cooling ("monotropic" liquid crystalline) and was only observed in a narrow 13.degree. C. temperature range (from 93.degree. C. to 80.degree. C.). Hence, the "processing window" of these monomers which results in anisotropic thermoset resins is narrow and is only accessible on cooling. A broad window is not only desirable to facilitate advantageous processing, but also increases the number and type of curing agents which may be used, therefore advantageously expanding the versatility of the invention.
It is therefore desirable to provide anisotropic monomers which can be cured with a variety of curing agents to produce a range of useful products which exhibit self-reinforcing characteristics. It is furthermore desirable that the monomers be capable of forming an anisotropic phase when heated or cooled ("enantiotropic" liquid crystalline) that is maintained over a broad temperature range (wide processing window) so that mixtures of the monomers and various curing agents well known to those skilled in the art can be employed.